There are ongoing discussions about introduction of an ultra-small wireless base station (hereinafter called “femto base station (HNB: Home Node B)” in cellular systems typified by WCDMA (Wideband Code Division Multiple Access). By providing a femto base station in a building of an ordinary household or an office where the propagation environment is comparatively poor, it is possible to cover an area of a radius of several tens of meters or so and achieve faster wireless transmission in the area where the femtocell is set up.
Since existing cellular systems are presumed to use all operation frequency bands in urban areas, it is difficult to secure a dedicated frequency band for femto base stations, and it is therefore more likely that frequency sharing is required between femto base stations and existing macro base stations. In addition, it is likely to support a CSG (Closed Subscriber Group)-based access limiting function which allows only a party to provide a femto base station to perform communication using that femto base station.
If a femto base station is introduced in an existing cellular system based on these conditions, mutual downlink interference, which the femto base station gives to an existing macrocell user (MUE: Macro User Equipment), or which an existing macro base station gives to a femtocell user (HUE: Home User Equipment), becomes a problem. Especially, in HSDPA (High Speed Data Packet Access) transmission, in which a base station carries out downlink transmission at maximum power, interference is a significant problem. A user of a femto base station set up near a macro base station receives significant interference from the macro base station. Meanwhile, a macrocell user located near a femto base station set up near a cell edge of a macro base station receives significant interference from the femto base station.
Non-Patent Literature 1 discloses a downlink interference control method in a femto base station set up in a cellular system. This interference control method will be described below with reference to FIG. 1. FIG. 1 shows the configuration of a wireless system where femto base stations are set up in the cover area of a macro base station.
According to the interference control method disclosed in Non-Patent Literature 1, as shown in FIG. 1, each femto base station 20-1 or 20-2 measures the reception power level of a common pilot signal (CPICH: Common Pilot Channel) transmitted from macro base station 10 to calculate wave pathloss (pathloss+shadowing) between macro base station 10 and femto base station 20-1 or wave pathloss between macro base station 10 and femto base station 20-2. The total downlink transmission power of each femto base station is decided from the pathloss between macro base station 10 and femto base station 20-1 or from the pathloss between macro base station 10 and femto base station 20-2, according to the total downlink transmission power determining function of femto base station 20-1 or femto base station 20-2 shown in FIG. 2. According to the method, a femto base station executes a control such that the closer to a macro base station a femto base station is located, the higher femto base station transmission power is increased, and the farther from a macro base station a femto base station is located, the lower femto base station transmission power is decreased.
Next, the operational steps of the interference control method disclosed in Non-Patent Literature 1 will be described below with reference to FIG. 3. In FIG. 3, a peripheral cell search is started in step ST31. In this peripheral cell search, a peripheral cell is determined to be present at a timing a correlation value to exceed a predetermined threshold is found as a result of synchronous channel correlation calculation.
The reception power of the CPICH from the detected cell is measured in step ST32, and the scrambling code of the detected cell is identified to decode broadcast information (BCH) in step ST33.
In step ST34, the transmission power value of the detected cell is acquired from the decoded broadcast information. When the acquired transmission power value is equal to or greater than a predetermined threshold, the detected cell is determined to be a macrocell, and the flow proceeds to step ST35. When the acquired transmission power value is less than the predetermined threshold, the detected cell is determined to be a femtocell, and the flow proceeds to step ST36.
In step ST35, when a reception power value in the detected macrocell is found to exceed the maximum value held, the maximum value is updated, and the macro base station of the strongest reception power and its reception power value are held.
In step ST36, whether the processes of steps ST32 through step ST35 are finished with respect to all timings where a synchronous channel correlation calculation result to exceed a predetermined threshold is found, and, if these processes are finished for all timings, a peripheral cell search is finished, or, if these processes are not finished for all timings, a peripheral cell search continues being repeated.
In step ST37, the set value of total downlink transmission power is calculated based on the measured strongest CPICH reception power value of peripheral cells. To be more specific, as shown in FIG. 2, the greater CPICH reception power is, the higher the total downlink transmission power value is set in a range between the maximum value (Pmax1) and the minimum value (Pmin1) of the values of power that can be transmitted by a femto base station, and the lower CPICH reception. power is, the lower the total downlink transmission power is set in that range.
By executing such a total downlink transmission power control for a femto base station, the following advantages are achieved. Since the desirable wave power for a femtocell user who has received significant interference from a macro base station near the macro base station is increased, the SIR (Signal to Interference Ratio) of the femtocell user can be improved. Since interference power on a macrocell user who has received significant interference from a femto base station is reduced at a macrocell edge, the SIR of the macrocell user can be improved.